High aspect ratio trench structures with void-free fill material

ABSTRACT

A field effect transistor (FET) includes a trench extending into a semiconductor region. A conductive electrode is disposed in the trench, and the conductive electrode is insulated from the semiconductor region by a dielectric layer. The conductive electrode includes a conductive liner lining the dielectric layer along opposite sidewalls of the trench. The conductive liner has tapered edges such that a thickness of the conductive liner gradually increases from a top surface of the conductive electrode to a point in lower half of the conductive electrode. The conductive electrode further includes a conductive fill material sandwiched by the conductive liner. The FET further includes a drift region of a first conductivity type in the semiconductor region, and a body region of a second conductivity type extending over the drift region. Source regions of the first conductivity type extend in the body region adjacent the trench.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/021,294, filed on Jan. 15, 2008, which is incorporated herein byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates in general to semiconductor technology,and more particularly to structures and methods for forming high aspectratio trenches. Merely by way of example, the invention has been appliedin a shielded gate trench field effect transistor (FET). But it would berecognized that the invention has a much broader range of applicability.

Shielded gate trench FETs and trench gate FETs are widely used in powerelectronics. In a shielded gate trench FET, the shield electrode reducesthe gate-drain capacitance (Cgd) and improves the breakdown voltage ofthe transistor without sacrificing the transistor on-resistance. FIG. 1is a simplified cross sectional view diagram of a conventional shieldedgate trench MOSFET. An n-type epitaxial layer 102 extends over n+substrate 101. N+ source regions 108 and p+ heavy body regions 106 areformed in a p-type body region 104 which is in turn formed in epitaxiallayer 102. Trench 110 extends through body region 104 and terminates inthe drift region which is the portion of epitaxial layer 102 extendingbetween body region 104 and substrate 100. Trench 110 includes a shieldelectrode 114 below a gate electrode 122. Gate electrode 122 isinsulated from its adjacent silicon regions by gate dielectric 120, andshield electrode 114 is insulated from its adjacent silicon regions by ashield dielectric 112 which is thicker than gate dielectric 120. Thegate and shield electrodes are insulated from each other by a dielectriclayer 116 also referred to as inter-electrode dielectric or IED.

For many applications a key performance characteristic of the trench FETis its switching speed. To maximize the switching speed of the trenchFET it is desirable to minimize the resistivity of its gate material. Asshown in FIG. 1, both the shield electrode 114 and the gate electrode122 are formed inside trench 110. With the advancement to technology,device size continues to shrink, and the aspect ratio of trenchstructures and recesses continues to increase. As a result, conventionalmethods for forming trench structures and recesses in general suffermany limitations. Some of the limitations are illustrated in FIGS. 2Aand 2B and described in more detail further below.

Thus, there is a need for simple and cost effective techniques forfilling high aspect ratio trenches and recesses in a void-free manner.BRIEF SUMMARY OF THE INVENTION

The present invention relates in general to semiconductor technology,and more particularly to methods and structures for high aspect ratiotrenches. Merely by way of example, the invention has been applied to amethod for void free filling of trenches and recesses with conductivematerials. In a specific embodiment, the trench or recess is firstpartially filled with a first conductive material which is etched backsuch that the remaining portion of the first conductive material hassidewalls with a positive slope. A second conductive material is thenused to fill the trench or recess such that the trench is substantiallyvoid free. A method is also provided for filling a trench with reentrantsidewalls. These and other embodiments will be briefly described next.

In accordance with one embodiment of the invention, a field effecttransistor (FET) includes a trench extending into a semiconductorregion. A conductive electrode is disposed in the trench, and theconductive electrode is insulated from the semiconductor region by adielectric layer. The conductive electrode includes a conductive linerlining the dielectric layer along opposite sidewalls of the trench. Theconductive liner has tapered edges such that a thickness of theconductive liner gradually increases from a top surface of theconductive electrode to a point in lower half of the conductiveelectrode. The conductive electrode further includes a conductive fillmaterial sandwiched by the conductive liner. The FET further includes adrift region of a first conductivity type in the semiconductor region,and a body region of a second conductivity type extending over the driftregion. Source regions of the first conductivity type extend in the bodyregion adjacent the trench.

In one embodiment, both the conductive liner and the conductive fillmaterial comprise polysilicon.

In another embodiment, the conductive liner comprises polysilicon andthe conductive fill material comprises metal-containing material.

In another embodiment, the conductive electrode is a gate electrode andthe dielectric layer is a gate dielectric layer.

In another embodiment, the trench further includes a thick bottomdielectric extending along the bottom of the trench directly below thegate electrode.

In another embodiment, the conductive liner is discontinuous along abottom of the conductive electrode so that the conductive linercomprises discrete conductive spacers extending over the dielectriclayer along opposite sidewalls of the trench.

In another embodiment, the conductive liner extending along oppositesidewalls of the trench is continuous along a bottom of the conductiveelectrode.

In another embodiment, the conductive electrode is a shield electrodedisposed in a lower portion of the trench, and the dielectric layer is ashield dielectric layer lining lower trench sidewalls.

In another embodiment, a gate dielectric layer lines upper trenchsidewalls. A gate electrode is disposed in the trench over the shieldelectrode, and an inter-electrode dielectric layer extends laterallybetween the gate and shield electrodes.

In another embodiment, the gate electrode includes a conductive linerlining the gate dielectric layer, and the conductive liner has taperededges such that a thickness of the conductive liner gradually increasesfrom a top surface of the gate electrode to a point in lower half of thegate electrode. A conductive fill material in the trench is sandwichedby the conductive liner.

In another embodiment, the top surface of the conductive electrode isnon-planar. In one variation, the fill material protrudes above theconductive liner, and in another variation, the fill material isrecessed relative to the conductive liner. In still another embodiment,the top surface of the conductive electrode is substantially planar.

In accordance with another embodiment, a field effect transistor (FET)includes a semiconductor region comprising a drift region of a firstconductivity type and a body region of a second conductivity typeextending over the drift region. The FET further includes a gateelectrode insulated from the body region by a gate dielectric layer.Source regions of the first conductivity type extend in the body region.A heavy body recess extends in the body region, and includes aconductive liner lining opposite sidewalls of the heavy body recess. Theconductive liner has tapered edges such that a thickness of theconductive liner gradually increases from the top of the heavy bodyrecess to a point in lower half of the heavy body recess. A conductivefill material fills a center portion of the heavy body recess and issandwiched by the conductive liner.

In one embodiment, the gate electrode is disposed in a trench extendingthrough the body region and into the drift region.

In another embodiment, the gate electrode is a planar gate laterallyextending over the semiconductor region.

In another embodiment, the conductive liner is discontinuous along abottom of the heavy body recess so that the conductive liner comprisesdiscrete conductive spacers extending along opposite sidewalls of theheavy body recess.

In another embodiment, the conductive liner extending along oppositesidewalls of the heavy body recess is continuous along a bottom of theheavy body recess.

In another embodiment, a heavy body implant region extends in the bodyregion along a bottom of the heavy body recess, and the conductive lineris in direct contact with the heavy body implant region along the bottomof the heavy body recess.

In accordance with yet another embodiment of the invention, a method forforming a trench gate field effect transistor includes forming a trenchin a semiconductor region. A conductive electrode is formed in thetrench. The conductive electrode is insulated from the semiconductorregion by a dielectric layer, the step of forming a conductive electrodeincludes: forming a conductive liner lining the dielectric layer alongopposite sidewalls of the trench, the conductive liner having taperededges such that a thickness of the conductive liner gradually increasesfrom a top surface of the conductive electrode to a point in lower halfof the conductive electrode; and forming a conductive fill materialfilling an opening formed by the conductive liner. A body region of afirst conductivity type is formed in the semiconductor region, andsource regions of the first conductivity type are formed in the bodyregion.

In one embodiment, both the conductive liner and the conductive fillmaterial comprise polysilicon.

In another embodiment, the conductive liner comprises polysilicon andthe conductive fill material comprises metal-containing material.

In another embodiment, the conductive electrode is a gate electrode andthe dielectric layer is a gate dielectric layer.

In another embodiment, the method further includes: before forming thegate electrode, forming a thick bottom dielectric extending along thebottom of the trench.

In another embodiment, the conductive liner is discontinuous along abottom of the conductive electrode so that the conductive linercomprises discrete conductive spacers extending over the dielectriclayer along opposite sidewalls of the trench.

In another embodiment, the conductive liner extending along oppositesidewalls of the trench is continuous along a bottom of the conductiveelectrode.

In another embodiment, the conductive electrode is a shield electrodedisposed in a lower portion of the trench, and the dielectric layer is ashield dielectric layer lining lower trench sidewalls.

In another embodiment the method further includes: forming a gatedielectric layer lining upper trench sidewalls, the shield dielectriclayer being thicker than the gate dielectric layer; and forming a gateelectrode disposed in the trench over the shield electrode, the gate andshield electrode being insulated from one another by an inter-electrodedielectric layer.

In another embodiment, the step of forming a gate electrode includes:forming a conductive liner lining the gate dielectric layer, theconductive liner of the gate electrode having tapered edges such that athickness of the conductive liner of the gate electrode graduallyincreases from a top surface of the gate electrode to a point in lowerhalf of the gate electrode, and forming a conductive fill materialfilling an opening formed by the conductive liner of the gate electrode.

In another embodiment, the conductive liner is formed using a bevel etchprocess or an anisotropic etch process.

Various additional features and advantages of the present invention canbe more fully appreciated with reference to the detailed description andaccompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view illustrating a conventionalshielded gate MOSFET;

FIGS. 2A and 2B are simplified cross-sectional views illustratingconventional trench filling and etch back methods;

FIGS. 3A-3D are simplified cross-sectional views illustrating a methodfor forming a trench structure according to an embodiment of the presentinvention;

FIGS. 4A-4D are simplified cross-sectional views illustrating a methodfor forming a trench structure with a reentrant trench profile accordingto another embodiment of the present invention;

FIGS. 5A-5I are simplified cross-sectional views illustrating a methodfor forming a shielded gate MOSFET according to an embodiment of thepresent invention;

FIG. 6 is a simplified cross-sectional view illustrating a trench gateMOSFET with heavy body recesses formed according to another embodimentof the present invention; and

FIG. 7 is a simplified cross-sectional view illustrating a trench gateMOSFET with gate electrode, shield electrode, and heavy body recessesformed according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in general to semiconductor technology,and more particularly to structures and methods for forming high aspectratio trenches and recesses. Merely by way of example, the invention hasbeen applied to a method for void free filling of a trench withconductive materials in forming the shield electrode in shielded gatetrench field effect transistors (FETs). In a specific embodiment, thetrench is first partially filled with a first conductive material, whichis etched back to provide a positive sidewall slope. A second conductivematerial is then used to fill the trench such that the trench issubstantially void free. A method is also provided for filling a trenchwith reentrant sidewalls.

In forming a shielded gate FET, a conductive material is used to fill atrench and then etched back to thereby form a shield electrode in abottom portion of the trench. FIG. 2A illustrates a conventional trenchfilling method. A trench 210 is formed in substrate 200. A dielectriclayer 212 is formed lining the sidewall and bottom surface of thetrench. Then a conductive material 220, such as polysilicon, isdeposited to fill the trench. As shown in FIG. 2A, even though the topportion of the trench is filled with conductive material 220, a void 230is formed inside the trench. In a chemical vapor deposition (CVD) orphysical vapor deposition (PVD) process, the deposited material often isnot conformal. As the aspect ratio of the trench becomes higher, itbecomes more difficult to fill a trench without a void. In FIG. 2B, theconductive material 220 is etched back in a process similar to that usedin forming a shielded gate. It can be seen that the remaining conductivematerial 221 includes the void 230. Such a void is undesirable for manyreasons. For example, the void can cause an increase in the resistanceof the conductive material and degrade device performance. It can alsotrap particles or contaminants and cause reliability problems.

A method of fabricating a trench structure according to an embodiment ofthe present invention will be described hereinafter using thecross-sectional views shown in FIGS. 3A-3D and the flow chart of FIG.3E. Referring to FIG. 3A, a trench 310 is formed in a semiconductorregion 301, for example, an epitaxial layer that extends over asemiconductor substrate (not shown). A dielectric layer 320 is formedlining the trench sidewalls and bottom and extending over mesa regionsadjacent the trench. Dielectric layer 320 is typically made of silicondioxide but could be made of other dielectric material such as anitride, or oxynitride. It is to be understood that any one of a numberof known trench processes can be employed for performing these orsimilar steps to prepare the structure up to this point.

Referring to FIG. 3B, after the formation of dielectric layer 320, alayer of conductive material 330 is deposited over the substrate topartially fill trench 310. As shown, conductive material 330 overliesdielectric material 320 and partially fills the trench. In a specificembodiment, the thickness of conductive material 330 is less than halfthe width of the trench opening, so the trench is partially filled.

Conductive material 330 can be any material having relatively lowresistivity and, in one embodiment, comprises a material which canwithstand high processing temperature. For example, conductive material330 can be doped polysilicon. Alternatively, a refractive metal such asW, Ti, Co, or a metal silicide such as WSi₂, TiSi₂ may be used. It isunderstood that as used herein the term “polysilicon” includespolysilicon and amorphous silicon. Polysilicon layer 330 can be dopedusing conventional doping processes such as POCl₃for n-type poly, p-type(e.g., boron) or n-type (e.g., phosphorous) implant for p or n typepoly, respectively, or in-situ doping of n or p type dopants.

Next, in FIG. 3C, a portion of polysilicon layer 330 is removed suchthat remaining polysilicon liner 331 extends from top corners of thetrench toward the bottom of the trench and provides positive sidewallslopes 333, 334 in the remainder of the trench. A bevel etch process maybe used. As can be seen, the positive sidewall slopes provide a wideropening at the top of the trench than near the bottom of the trench.

A conventional anisotropic etch process, such as a reactive ion etch(RIE) process, can be used. Alternatively, a combination of anisotropicand isotropic etch processes can be combined to tailor the contours ofconductive liner 331. As shown, the polysilicon is fully removed fromover the mesa regions adjacent the trench. In an alternative embodiment,positive sidewall slopes can also be obtained with polysilicon partiallyremaining on the mesa regions.

Next, in FIG. 3D, a layer of high-conductivity material 340 is depositedover conductive liner 331 to substantially fill the remainder of thetrench. The layer of high-conductivity material 340 may be dopedpolysilicon or any type of refractory metal such as tungsten, tungstensilicide, titanium, cobalt, platinum, or the like. A polysilicon layercan be formed using a low-pressure chemical vapor deposition (LPCVD)process. Refractive metals can be deposited using a chemical vapordeposition (CVD) or a physical vapor deposition (PVD) process.

In a specific embodiment, the method may also include removing a nativeoxide layer from an exposed surface of the first conductive materialprior to substantially filling the remainder of the trench with thesecond conductive material. Alternatively, after forming highlyconductive material 340, heat cycles may be used to allow the nativeoxide to absorb into poly liner 331 and/or highly conductive material340. Native oxides can form over the surface of conductive liner 331 andcan deteriorate the contact between the conductive spacers and secondconductive material 340. In certain applications, it may be advantageousto include a native oxide removal process in the method.

As can be seen in FIG. 3D, trench 310 is filled with conductive material340 and conductive liner 331. The wider trench opening provided byconductive liner 331 enables trench 310 to be filled in a substantiallyvoid free manner. The structure in FIG. 3E can be used in further deviceprocessing. Some examples will be provided below.

The method of FIGS. 3A-3D can also be used for filling a trench having areentrant profile, which makes trench filling even more difficult. FIGS.4A-4D are simplified cross section views illustrating a method forfilling a reentrant trench according to another embodiment of thepresent invention. As shown, trench 410 in FIG. 4A has a reentrantprofile, that is, the trench sidewalls have negative slopes resulting ina narrower opening at the top of the trench and wider spacing inside thetrench. The method depicted in FIGS. 4A-4D is similar to the method ofFIGS. 3A-3D. Trench 410 is first partially filled with a firstconductive material 430 which has sidewalls 424 and 425 having negativeslopes. Parts of conductive material 430 are removed, and conductiveliner 431 is formed. Conductive liner 431 has positive sidewall slopes.A wider opening is provided which allows a second conductive material440 to be formed thus providing substantially void free filling of thetrench. In the case where the trench sidewalls have relatively largenegative slopes, it may be necessary to repeat the steps of depositingand etching back a conductive material (such as polysilicon) a number oftimes to ensure that the no voids are formed.

The techniques described above in connection with FIGS. 3A-3E and FIGS.4A-4D can be used in forming trench gate and shielded gate trenchtransistors, as discussed next. FIGS. 5A-5G are simplifiedcross-sectional views illustrating a process flow for forming a shieldedgate trench field effect transistor according to an embodiment of thepresent invention. The following description of the steps in the processflow is only exemplary, and it should be understood that the scope ofthe invention is not limited to this specific example. In particular,processing conditions such as temperature, pressure, and layerthicknesses could be varied without departing from the spirit of theinvention.

In FIG. 5A, using conventional techniques, trench 510 is formed in asemiconductor region, for example, a silicon region. In one embodimentthe semiconductor region includes a highly doped substrate 501 (e.g.,n-type in case of an n-channel MOSFET) and an epitaxial layer 502 (e.g.,n-type in case of an n-channel MOSFET) overlying substrate 510. Trench510 extends into and terminates within epitaxial layer 502. In anothervariation, trench 510 extends through the epitaxial layer and terminateswithin the substrate. Note that the various dimensions in the figures ofthis application are not to scale and at times are exaggerated orreduced in size to more clearly show various structural features.

In FIG. 5A, shield dielectric layer 511 is formed lining the trenchsidewalls and bottom and extending over mesa regions adjacent thetrench, using conventional techniques. In one embodiment, shielddielectric layer 511 is formed to the desired thickness using thermaloxidation of silicon. The steps depicted in FIGS. 5B and 5C for formingconductive liner 531 are similar to those depicted in FIGS. 4B and 4Cdescribed above, and thus will not be described.

In FIGS. 5D-5E, a shield electrode is formed in a bottom portion oftrench 510. As shown in FIG. 5D, using known techniques, a secondconductive material 540 (e.g., comprising doped or undoped polysiliconor various metal-containing materials mentioned above) is formed to fillthe trench and extending over the mesa regions. Subsequently, as shownin FIG. 5E, conductive liner 531 and conductive fill material 540 arerecessed deep into trench 510 to form shield electrode 514. Shieldelectrode 514 thus includes portion 534 remaining from conductive liner531 and portion 535 remaining from second conductive material 540. Asshown, portion 535 is sandwiched by portion 534. One or multiple etchprocesses may be necessary depending on whether conductive liner 531 andconductive fill material 540 are of the same or different material, andwhether a shield electrode with a planar or non-planar surface isdesired. For example, it may be desirable to recess fill material 540deeper than liner 531 or vice versa depending on the desiredcharacteristics of the shield electrode itself or of the subsequentmaterials formed in the trench (e.g., IED 516, gate dielectric 520 andgate electrode 522).

In FIG. 5F, using known dielectric etch methods, exposed portions ofshield dielectric layer 511 along upper trench sidewalls and over mesasurfaces are removed. The remaining portion of the shield dielectric islabeled 512. In FIG. 5G, an inter-electrode dielectric (IED) 516 isformed over shield electrode 514. In one embodiment, polysilicon shieldelectrode 514 is oxidized using a conventional oxidation process to formIED 516. In an alternate embodiment, a dielectric layer is depositedover shield electrode 514. In still another embodiment, a combination ofoxidation and dielectric deposition are used to form IED 516. Gatedielectric 520 is then formed lining upper trench sidewalls using knownmethods. In an alternate embodiment, the process for forming gatedielectric 520 can be used to also form IED 516 (i.e., eliminateseparate processing for forming IED 516).

Using conventional techniques (not shown), a second conductive layer(e.g., comprising doped polysilicon or a metal-containing material suchas silicide or refractory metal) is formed filling trench 510 andextending over the mesa surfaces. The second conductive material is thenrecessed in trench 510 thus forming the gate electrode. In an alternateembodiment, the gate electrode is formed using the method of FIGS. 3A-3Dor 5A-5D. An example of this embodiment is shown in FIG. 5G-5H, in whicha first conductive material is deposited and partially removed to formconductive liner 541, and then a second conductive material 545 isdeposited to fill the remaining portion of the trench. Liner 541 andsecond conductive material 545 are then recessed into trench 510 to formgate electrode 522, as shown in FIG. 5I. Void-free gate and shiedelectrodes can thus be formed.

Any number of known process steps may next be carried out to completethe trench FET structure. FIG. 5I shows one exemplary trench gate MOSFETstructure 500. P-type body region 504, n-type source regions 508, andp-type heavy body regions 506 are formed in n-type epitaxial layer 502using conventional implanting and drive-in techniques. The portion ofepitaxial layer 502 bounded by body region 304 and substrate 501 iscommonly referred to as the drift region and is lighter doped thansubstrate 501. Dielectric cap 524 (e.g., comprising BPSG) is formed overgate electrode 522 using known techniques. Top-side interconnect layer526 (e.g., comprising metal) is formed to electrically contact sourceregions 508 and heavy body regions 506. Back-side drain interconnect(not shown) is formed on the back side to electrically contact heavilydoped n-type substrate 501 using known methods. The back-side draininterconnect can be formed using a suitable conductor, e.g., a metal.

Note in FIG. 5I, both shield electrode 514 and gate electrode 522 areformed using the method described above in connection with FIGS. 3A-3Eor 5A-5H. In this embodiment, shield electrode 514 is shown to have tworegions: region 534 is formed using a first conductive material, such aspolysilicon, and region 535 is formed using a second conductive materialwhich can be polysilicon or a metal-containing material such as silicideor a refractory metal. Similarly, gate electrode 522 also has tworegions: region 541 is formed using a first conductive material, such aspolysilicon, and region 545 is formed using a second conductive materialwhich can be polysilicon or a metal-containing material such as silicideor a refractory metal. In other embodiments, only one of shieldelectrode 514 or gate electrode 522 can be formed using the methoddescribed above while the other electrode is formed using conventionaltechniques.

In FIGS. 5A-5I, the trench structure is formed as part of a trenchtransistor in a process in which forming the trench and the materialtherein occurs before the doped junctions (e.g., well, source regionsand heavy body regions) are formed. In such embodiment, usingpolysilicon for both conductive regions of each of the gate and shieldelectrodes is preferred over a metallic compound given the hightemperature cycles associated with the implant and drive in of junctionssuch as the well and source regions. In another embodiment, the trenchstructure and the material therein are formed after the well implant anddrive-in.

As discussed above, the present invention provides a method for forminga shielded gate FET in which a first conductor partially filling thetrench is etched back to provide positive sidewall slopes. A secondconductive material is then used to fill the trench such that the trenchis substantially void free. The conductive materials are then etched toform the shield electrode. The method can also be used in forming thegate electrode.

FIG. 6 is a cross-sectional view diagram illustrating a trench gateMOSFET 600 according to another embodiment of the present invention.Trench gate MOSFET 600 is similar to trench gate transistor 500 in FIG.5. As shown, trench gate MOSFET 600 includes a trench 610 extending intodrift region 602, with gate electrode 622 and shield electrode 614disposed in the trench. The shield and gate electrodes are insulatedfrom the surrounding semiconductor region by shield dielectric 612 andgate dielectric 620, respectively. Trench gate MOSFET 600 also includesbody region 604 of p-type conductivity extending over drift region 602.Source regions 608 of n-type conductivity type extend in body region 604adjacent to the trench.

Additionally, MOSFET 600 has a heavy body contact region 607, whichincludes a heavily doped p-type diffusion region 606 and recess regions625 filled with conductive materials. As the lateral cell pitchcontinues to shrink, the aspect ratio of heavy-body recesses increases,thus making it more difficult to fill the recesses in a void-freemanner. In one embodiment, the methods described above can be used forvoid-free filling of recesses 625. In the embodiment shown, reach recessregion 625 includes conductive liner 627 made of a first conductivematerial and a center region 629 made of a second conductive materialsandwiched by conductive liner 627. Depending on the embodiment, variousconductive materials can be used. For example, conductive liner 627 andsecond conductive material may comprise heavily doped polysilicon and/ora metal-containing material such as silicide or a refractory metal suchas W, Ti, etc. Alternatively, center region 625 may form part oftop-side source interconnect 626 which can comprise aluminum or ametal-containing material such as silicide or a refractory metal such asW and Ti.

In the embodiments described above, the conductive liner sandwiching thecenter conductive material in the trench or in the heavy body recess wasetched such that a thickness of the conductive liner gradually increasesfrom top to bottom with the conductive liner fully covering the bottomof the trench or recess. However, in some embodiments, depending on thedepth of the opening and the type of etch used, a portion of theconductive liner extending along the bottom of the opening may becompletely removed thereby forming discrete conductive spacers alongsidewalls of the opening with a bottom surface of the opening becomingexposed. FIG. 7 is a cross section view showing an embodiment whereinsuch spacers are formed. The shielded MOSFET shown in FIG. 7 issubstantially similar to that in FIG. 6 except that shield electrode714, gate electrode 722 and heavy body recess contact 726 have adifferent structure. Each of shield electrode 714, gate electrode 722and heavy body recess contact 726 includes conductive spacers thatextend along sidewalls of the respective openings but are discontinuousalong the bottom of the openings. Thus, each of shield electrode 714,gate electrode 722 and heavy body recess contact 726 includes threedistinct regions: shield electrode 714 includes outer spacers 714 andcentral conductive region 735; gate electrode 722 includes outer spacers741 and central conductive region 745; and heavy body contact includesouter spacers 727 and central conductive region 729.

Many benefits are achieved by various embodiments of the invention. Forexample, gate and/or shield electrodes are formed substantially free ofvoids which can trap particles or contaminants and cause reliabilityproblems. Voids can also cause an increase in the resistance of shieldelectrode and gate electrode and degrade device performance. For manyapplications, a key performance characteristic of the trench gate FET isits switching speed. To maximize the switching speed of the trench gateFET, it is desirable to minimize the resistivity of its gate material.Accordingly, the methods provided by embodiments of the invention canaid in improving the performance and reliability of trench gate andshielded gate FETs. Additionally, the present techniques are implementedusing simple and cost effective processes which can be readilyintegrated with conventional process technologies. The processes, inaccordance with embodiments of the invention, are compatible withconventional process technology without the need for any significantmodifications to conventional equipment and processes. Depending uponthe embodiment, one or more of these benefits may be obtained. These andother benefits will be described in more detail throughout the presentspecification and more particularly below.

The above description is directed to n-channel shielded gate FETsaccording to specific embodiments of the present invention. However, thesame techniques can apply to other types of shielded gate trench FETs.For example, while embodiments of the invention are described in thecontext of n-channel MOSFETs, the principles of the invention may beapplied to p-channel MOSFETs by merely reversing the conductivity typeof the various regions. Additionally, the principle of the invention canalso be applied to shielded gate IGBTs by merely reversing theconductivity of the substrate in the above-described embodiments. Forinstance, by merely changing the conductivity type of substrate 501 inFIG. 5I from n-type to p-type, n-channel IGBT counterpart of the MOSFETsin FIG. 5I is obtained with the same advantages outlined above.Additionally, P-channel IGBTs can be obtained by reversing theconductivity type of the various regions except for the substrate.Furthermore, the MOSFET and IGBT embodiments referenced herein arevertically conducting devices (i.e., the source and drain interconnectsare on opposite surfaces of the device), the techniques according to theinvention can also be applied to laterally conducting trench gateMOSFETs and IGBTs (i.e., where the source and drain contacts are bothmade along the top surface of the device).

While the above provides a complete description of the preferredembodiments of the invention, many alternatives, modifications, andequivalents are possible. Those skilled in the art will appreciate thatthe same techniques can be used in other applications. Therefore, theabove description should not be taken as limiting the scope of theinvention, which is defined by the appended claims.

1. A trench gate field effect transistor, comprising: a trench extendinginto a semiconductor region; a conductive electrode disposed in thetrench, the conductive electrode being insulated from the semiconductorregion by a dielectric layer, the conductive electrode including aconductive liner lining the dielectric layer along opposite sidewalls ofthe trench, the conductive liner having tapered edges such that athickness of the conductive liner gradually increases from a top surfaceof the conductive electrode to a point in lower half of the conductiveelectrode, the conductive electrode further including a conductive fillmaterial sandwiched by the conductive liner; a drift region of a firstconductivity type in the semiconductor region; a body region of a secondconductivity type extending over the drift region; and source regions ofthe first conductivity type in the body region adjacent the trench. 2.The field effect transistor of claim 1 wherein both the conductive linerand the conductive fill material comprise polysilicon.
 3. The fieldeffect transistor of claim 1 wherein the conductive liner comprisespolysilicon and the conductive fill material comprises metal-containingmaterial.
 4. The field effect transistor of claim 1 wherein theconductive electrode is a gate electrode and the dielectric layer is agate dielectric layer.
 5. The field effect transistor of claim 4 whereinthe trench further includes a thick bottom dielectric extending alongthe bottom of the trench directly below the gate electrode.
 6. The fieldeffect transistor of claim 1 wherein the conductive liner isdiscontinuous along a bottom of the conductive electrode so that theconductive liner comprises discrete conductive spacers extending overthe dielectric layer along opposite sidewalls of the trench.
 7. Thefield effect transistor of claim 1 wherein the conductive linerextending along opposite sidewalls of the trench is continuous along abottom of the conductive electrode.
 8. The field effect transistor ofclaim 1 wherein the conductive electrode is a shield electrode disposedin a lower portion of the trench, and the dielectric layer is a shielddielectric layer lining lower trench sidewalls.
 9. The field effecttransistor of claim 8 further comprising: a gate dielectric layer liningupper trench sidewalls, the shield dielectric layer being thicker thanthe gate dielectric layer; a gate electrode disposed in the trench overthe shield electrode; and inter-electrode dielectric layer extendinglaterally between the gate and shield electrodes.
 10. The field effecttransistor of claim 9 wherein the gate electrode comprises: a conductiveliner lining the gate dielectric layer, the conductive liner of the gateelectrode having tapered edges such that a thickness of the conductiveliner of the gate electrode gradually increases from a top surface ofthe gate electrode to a point in lower half of the gate electrode, and aconductive fill material sandwiched by the conductive liner of the gateelectrode.
 11. The field effect transistor of claim 1 wherein the topsurface of the conductive electrode is non-planar.
 12. A field effecttransistor, comprising: a semiconductor region comprising a drift regionof a first conductivity type and a body region of a second conductivitytype extending over the drift region; a gate electrode insulated fromthe body region by a gate dielectric layer; source regions of the firstconductivity type in the body region; and a heavy body recess extendingin the body region, the heavy body recess comprising: a conductive linerlining opposite sidewalls of the heavy body recess, the conductive linerhaving tapered edges such that a thickness of the conductive linergradually increases from the top of the heavy body recess to a point inlower half of the heavy body recess, and a conductive fill materialfilling a center portion of the heavy body recess and sandwiched by theconductive liner.
 13. The field effect transistor of claim 12 whereinboth the conductive liner and the conductive fill material comprisehighly doped polysilicon.
 14. The field effect transistor of claim 12wherein the conductive liner comprises doped polysilicon and theconductive fill material comprises metal-containing material.
 15. Thefield effect transistor of claim 12 wherein the gate electrode isdisposed in a trench extending through the body region and into thedrift region.
 16. The field effect transistor of claim 12 wherein thegate electrode is a planar gate laterally extending over thesemiconductor region.
 17. The field effect transistor of claim 12wherein the conductive liner is discontinuous along a bottom of theheavy body recess so that the conductive liner comprises discreteconductive spacers extending along opposite sidewalls of the heavy bodyrecess.
 18. The field effect transistor of claim 12 wherein theconductive liner extending along opposite sidewalls of the heavy bodyrecess is continuous along a bottom of the heavy body recess.
 19. Thefield effect transistor of claim 12 further comprising a heavy bodyimplant region extending in the body region along a bottom of the heavybody recess, the conductive liner being in direct contact with the heavybody implant region along the bottom of the heavy body recess.
 20. Amethod of forming a trench gate field effect transistor, comprising:forming a trench in a semiconductor region; forming a conductiveelectrode in the trench, the conductive electrode being insulated fromthe semiconductor region by a dielectric layer, the step of forming aconductive electrode comprising: forming a conductive liner lining thedielectric layer along opposite sidewalls of the trench, the conductiveliner having tapered edges such that a thickness of the conductive linergradually increases from a top surface of the conductive electrode to apoint in lower half of the conductive electrode, and forming aconductive fill material in an opening formed by the conductive liner;forming a body region of a first conductivity type in the semiconductorregion; and forming source regions of the first conductivity type in thebody region.
 21. The method of claim 20 wherein both the conductiveliner and the conductive fill material comprise polysilicon.
 22. Themethod of claim 20 wherein the conductive liner comprises polysiliconand the conductive fill material comprises metal-containing material.23. The method of claim 20 wherein the conductive electrode is a gateelectrode and the dielectric layer is a gate dielectric layer.
 24. Themethod of claim 23 further comprising: before forming the gateelectrode, forming a thick bottom dielectric extending along the bottomof the trench.
 25. The method of claim 20 wherein the conductive lineris discontinuous along a bottom of the conductive electrode so that theconductive liner comprises discrete conductive spacers extending overthe dielectric layer along opposite sidewalls of the trench.
 26. Themethod of claim 20 wherein the conductive liner extending along oppositesidewalls of the trench is continuous along a bottom of the conductiveelectrode.
 27. The method of claim 20 wherein the conductive electrodeis a shield electrode disposed in a lower portion of the trench, and thedielectric layer is a shield dielectric layer lining lower trenchsidewalls.
 28. The method of claim 27 further comprising: forming a gatedielectric layer lining upper trench sidewalls, the shield dielectriclayer being thicker than the gate dielectric layer; and forming a gateelectrode disposed in the trench over the shield electrode, the gate andshield electrode being insulated from one another by an inter-electrodedielectric layer.
 29. The method of claim 28 wherein the step of forminga gate electrode comprises: forming a conductive liner lining the gatedielectric layer, the conductive liner of the gate electrode havingtapered edges such that a thickness of the conductive liner of the gateelectrode gradually increases from a top surface of the gate electrodeto a point in lower half of the gate electrode, and forming a conductivefill material filling an opening formed by the conductive liner of thegate electrode.
 30. The method of claim 20 where in the conductive lineris formed using a bevel etch process.
 31. The method of claim 20 whereinthe conductive liner is formed using an anisotropic etch process.